Protective coating for metals and method of making the same



United States Patent 3,240,572 PROTECTIVE COATING FOR METALS AND METHOD OF MAKING THE SAME Guy Faber, Rieden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie., Baden, Switzerland, a

joint-stock company No Drawing. Filed Feb. 8, 1963, Ser. No. 257,087

Claims priority, application Switzerland, Feb. 16, 1962, 1,919/62; Jan. 21, 1963, 683/63 3 Claims. (Cl. 29-1961) The invention relates to a method for the manufacture of a corrosion and heat resisting coating on metals, especially austenitic heat resistant steels and heat resistant nickel and cobalt alloys as well as to the protective coating manufactured in accordance with this method.

It is known to protect base metals such as iron or low alloyed steel against corrosion by means of a chromium coating. Often, in order to increase the protective effect, additional layers are applied between the chromium and the metal to be protected, usually noble metals such as copper and/or nickel. Protective coatings of this type have given good results against attack by aqueous corrosive agents at temperatures up to several hundred degrees centigrade. These coatings are not effective at temperatures above 600 C., especially with temperature fluctuations, because the coatings will flake off.

Therefore, the proposal has been made to protect iron or standard steel by means of a galvanically applied chromium coating and welding the coating solidly to the steel base by annealing at temperatures ranging from 1050 to 1150 C. In this manner a flaking off of the chromium coating can be definitely avoided, even if the material is later on subjected to temperatures between 600 and 850 C. and to fluctuations in temperature.

It has been found that not only iron and the low alloyed ferritic steels but also the rust resistant high alloyed austenitic steels as well as the so-called super-alloys of nickel and cobalt utilized for example for gas turbine blades can become subject to corrosion at temperatures above 600 0, especially if vanadium pentoxide or alkali metal sulfates are present. An obvious solution would be the protection of these steels, or super alloys respectively, by the application of a chromium coating and subsequent homogenizing. Many attempts were made to attain this aim but always unsuccessfully because it was found that the chromium coating will flake off, either immediately during the cooling-off period following the homogenizing process or later on when the material is heated to operating temperatures.

Attempts were then made to protect rust resistant austenitic steels and alloys of nickel and cobalt which are subjected to corrosive agents at high temperatures by concentrating chromium, silicon or aluminum at their surfaces, for example by diffusion from the gaseous phase, but coatings manufactured by this method contain at best 60% of the protective additional metal. Such coatings do provide a certain protection against corrosion but the protective effect is lower than in case of pure chromium.

It is also known to cover the basic material with a thin layer of nickel prior to the application of the chromium and the homogenizing process, thereby increasing the adhesive properties of the chromium coating. However, these intermediate nickel layers are not practical in the presence of sulfurous gases at temperatures above 650 C. Since the galvanically applied chromium coating always contains a network of very small cracks which will become still wider when the metal to be protected is heated, due to the low coefficient of expansion of chromium, sulfurous gases or combustion residues will have access to the layer of nickel and it is a well known fact that nickel will form low-melting compounds with sulphur. The

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liquid corrosion products so generated will not prevent the further attack by corrosive agents and the chromium coating will become undermined and its protective effect will be nullified.

The method of the present invention for the manufacture of a corrosion and heat resisting coating on metals, especially austenitic heat resistant steels and heat resistant nickel and cobalt alloys, avoids the above listed disadvantages of the known methods. In accordance with the present invention the metal to be protected is first coated with a metallic ferrous intermediate layer and then with a protective chromium coating and finally subjected at a temperature between 1050 and 1250 C. under neutral atmosphere to a homogenizing process.

A particularly advantageous intermediate layer between the metal to be protected and the chromium coating is plain iron which should be as free as feasible from silicon and carbon. When corrosive agents attack the intermediate layer of iron through the unavoidable cracks in the chromium coating solid and dense corrosion products are formed which close off the cracks and prevent any further penetration of the corrosive agent. Such intermediate layers of iron can be manufactured relatively easily and inexpensively. They are unsusceptible, for all practical purposes, to the influence of sulphur, and the chromium coating shows no tendency to flake off.

After extensive heating within a temperature range between 650 and 850 C. chromium will form an intermetallic connection with iron, a so-called sigma-phase which is somewhat brittle. This characteristic is quite noticeable at room temperature but has little significance during operations because this intermetallic phase has still suflicient toughness at elevated temperatures. In certain instances however, for example in case of frequently and rapidly starting gas turbines where great diflerences in temperature and consequently thermal stresses will occur between blade surface and core it is desirable to increase the toughness of the intermediate layer.

The formation of the brittle sigma phase can be avoided if the iron layer is alloyed with at least 25% nickel. In this case a higher sensitivity to the influence by sulphur upon the intermediate layer is intentionally accepted, a sensitivity which increases proportionally to the increase in the nickel content. Intermediate layers of iron-nickel alloy have favorable adhesive characteristics. They are slightly more costly than layers of iron but have practically no brittleness if the nickel content is increased above 25%. They can be utilized advantageously in cases where the sulphur content of the attacking agents is low.

In special circumstances where a great stability under changing temperatures as well as resistance to sulphur attack are required it is possible as has been proved by tests, to avoid the formation of the brittle sigma phase or at least delay it by some 10,000 hours when the intermediate layer of iron is alloyed with .5 to 5% of aluminum. The toughness of such intermediate layers is identical with the toughness of nickel-containing intermediate layers. In other words their brittleness is insignificant even when heated for long periods of time at a high operating temperature and the excellent adhesive properties shown by the intermediate layers made of iron are maintained. However, the process for the manufacture of such layer is somewhat more complicated and therefore more costly than in case of pure iron.

The intermediate layer as well as the chromium coating can be manufactured in a simple manner galvanically or by flame spray or by immersion in a suspension of the appropriate metal powder. Usually, the intermediate layer has a thickness of .01 to .1 mm., and the chromium J coating a thickness of .01 to .5 mm. An insuflicient thickness will impair the protective effect and an excessive thickness of the chromium coating Will lead to the formation of Wider cracks.

The intermediate layers which as such are much less scale-resistant than the metal to be protected and the upper chromium coating are connected very solidly with the basic material and to each other by the homogenizing process. In order to prevent any scaling the homogenizing process is carried out either under a neutral atmosphere or under high vacuum.

The method of the invention, i.e., the application of an intermediate layer, the application of a chromium coating upon such intermediate layer and homogenizing has the efiect, in contrast to the direct application of a chromium coating to the high-alloyed metal and homogenizing, that the chromium will have excellent adhesive properties and has no tendency to flake otf even under frequent changes in temperature.

I claim:

1. An article having a base of a metal selected from the group consisting of austenitic heat-resistant steels and heat-resistant nickel and cobalt alloys, an intermediate coating of plain iron substantially free of carbon and silicon and a top coating of chromium.

2. An article having a base of a metal selected from the group consisting of austenitic heat-resistant steels and References Cited by the Examiner UNITED STATES PATENTS 1,608,694 11/1926 Cain 29l96.6 1,746,987 2/1930 Bennett 29l96.6 X 1,896,411 2/1933 Maskrey 29l96.6 2,188,399 1/1940 Bieber 29l96.6 2,402,834 6/1946 Nachtm'an 20437 2,759,249 8/1956 Eberle 29l96.6 X 3,057,048 10/1962 Hirakis 204-37 X OTHER REFERENCES Metals Handbook, pages 553-556, published by American Society for Metals, copy 110, 1948 edition.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner. 

1. AN ARTICLE HAVING A BASE OF A METAL SELECTED FROM THE GROUP CONSISTING OF AUSTENITIC HEAT-RESISTANT STEELS AND HEAT-RESISTANT NICKEL AND COBALT ALLOYS, AN INTERMEDIATE COATING OF PLAIN IRON SUBSTANTIALLY FREE OF CARBON AND SILICON AND A TOP COATING OF CHROMIUM. 